Centrifugal fluid machines each having a centrifugal rotary impeller have been used in various plants, air-conditioning machines and liquid pressure-feed pumps. With the demand for environmental burden reduction growing higher in recent years, the centrifugal fluid machines are required to achieve higher efficiency and wider operating ranges than before.
An example of existing type of centrifugal fluid machine will be described in the following using FIG. 15. FIG. 15 is a sectional view on a plane crossing an impeller rotary axis of an existing type of centrifugal fluid machine. The existing type of centrifugal fluid machine mainly includes a centrifugal impeller 1 for providing a fluid with energy by means of rotation, a rotary shaft 2 for rotating the impeller, a diffuser 3 which, being located radially outside the impeller 1, converts the dynamic pressure of the fluid flowing in through the outlet of the impeller into a static pressure, and a return channel 4 which, being located downstream of the diffuser 3, leads the fluid to a downstream flow path 6. The impeller 1 is composed of a disk (hub) 11 coupled to a main shaft, a side plate (shroud) 12 facing the hub 11, and plural blades 13 circumferentially arranged between the hub 11 and the shroud 12. There are also cases in which an impeller having no shroud is used. The diffuser 3 is either a vaned diffuser having plural circumferentially arranged blades or a vaneless diffuser.
In the above centrifugal fluid machine, fluid is sucked in through an impeller inlet 5 and has its pressure increased by passing through the impeller 1, diffuser 3, and return channel 4 to be then led to the downstream flow path 6.
For efficiency enhancement of a centrifugal fluid machine, an impeller plays a very important role. To enhance the efficiency of an impeller, it is necessary to reduce losses such as friction loss generated on a wall surface when fluid flows inside the impeller, deceleration loss generated when the relative velocity of the fluid flowing in the impeller, from the impeller inlet toward the impeller outlet, decreases causing the boundary layer thickness of the flow near the wall surface to increase, and secondary flow loss generated when low velocity, low energy fluid flowing near the wall surface is driven by static pressure gradients in sectional planes perpendicularly intersecting with the main flow direction in the impeller.
Various methods have been proposed to reduce the secondary flow loss among the above-mentioned losses. PTL 1 listed in the following, for example, introduces an example method for reducing the secondary flow loss. In the method, the blade loading distribution on an impeller included in a centrifugal fluid machine is studied; the blade loading on the shroud side is made to concentrate on the leading edge side of each blade, and the blade loading on the hub side is made to concentrate on the trailing edge side of each blade, thereby reducing the static pressure difference between the hub and the shroud near the suction surface at the trailing edge on the shroud side of each blade (see FIG. 16 being described later) where fluid with low energy in particular tends to accumulate.
There are also examples like those described in PTL 1 to PTL 3 listed in the following in which the secondary flow loss is reduced by circumferentially inclining each blade such that, in a trailing edge portion of each blade, the hub side is ahead of the shroud side in the direction of impeller rotation. By shaping the trailing edge portion of each blade like this, the effect as illustrated in FIG. 16 (b) can be obtained. In FIG. 16, two adjacent blades of an impeller are shown with the shroud omitted. Blade force F applied from a pressure surface 14 of each blade 13 (leading-side surface of each blade in the direction of impeller rotation) to the fluid flowing in the impeller is directed perpendicularly to the pressure surface 14 of each blade. Therefore, in an impeller in which, as shown in FIG. 16 (a), each blade is inclined in a trailing edge portion thereof to be opposite to the blade inclination proposed in PTL 1 to PTL 3 (i.e. when the hub side of each blade is, in a trailing edge portion 17 thereof, behind the shroud side thereof in the direction of impeller rotation), the static pressure on the hub-side pressure surface 141 of each blade normally increases. This static pressure, however, decreases when each blade of the impeller is shaped as shown in FIG. 16 (b). On the other hand, the static pressure on the shroud-side suction surface 151 of each blade that normally decreases when each blade is shaped as shown in FIG. 16 (a) increases when each blade is shaped as shown in FIG. 16 (b). Therefore, the secondary flow that is, when each blade is shaped as shown in FIG. 16 (a), formed to accumulate low-energy fluid on the shroud-side suction surface 151 is suppressed when each blade is shaped as shown in FIG. 16 (b). The secondary flow loss is thus reduced.